Electrically small antennas have been the subject of many studies over the past few decades. An “electrically-small” antenna refers to an antenna or antenna element with relatively small geometrical dimensions compared to the wavelength of the electromagnetic fields the antenna radiates. In particular, a number of theoretical studies have examined the relationship between the electrical dimensions (physical dimensions normalized to the wavelength) of an antenna and its radiation characteristics including gain, radiation efficiency, bandwidth, and directional characteristics. These studies point to a set of either fundamental or practical limitations that govern the performance of such antennas. In particular, as the electrical dimensions of an antenna are decreased, the radiation efficiency and bandwidth also decrease. As a result, these studies propose a set of fundamental limits that predict the upper bounds of these radiation parameters.
Similar theoretical studies have been carried out to investigate the relationship between the directionality of an antenna array or continuous aperture and its electrical size. The results show that, in theory, achieving super-directivity is possible from an antenna array or a continuous aperture. In principle, such super-directive arrays can be used to precisely resolve the direction of arrival of an electromagnetic (EM) wave. However, when the overall electrical dimensions of the antenna array decrease, the nearby elements of the array must be excited with significantly oscillatory and widely varying excitation coefficients to achieve super-directional characteristics. Thus, though mathematically possible, the realization of such excitation coefficients is not practical for small antenna arrays due to problems such as mutual coupling between the elements and the tolerances required in device fabrication.
Studies have also been conducted in which the auditory system of the parasitic fly, Ormia Ochracea, has been analyzed due to the ability of the female fly to accurately resolve the call of the male fly despite the small distance between the ears of the female fly in relation to the wavelength of the call. Proposed antenna/signal processing designs based on the anatomy of the ears of the fly have shown enhanced resolution capabilities compared to regular antenna arrays that occupy the same area. In this context, resolution capability refers to the ability of the antenna array to detect the direction of arrival of an electromagnetic wave. However, the improved resolution capability has not translated to a higher gain or directivity for the antenna array. An inherent tradeoff exists between the angular sensitivity of the system and its output signal to noise ratio (SNR). In particular, as the angular sensitivity of the antenna increases, its output SNR decreases. This tradeoff exists in nature, where small animals that possess the sense of directional hearing have traded the capability to hear over long distances in favor of the capability of localizing the sound source of interest to a smaller area.